Abstract
Changes in chromosomal structure involving chromosomal rearrangements or copy number variation of specific sequences can play an important role in speciation. Here, we explored the chromosomal structure of two hybridizing passerine species; the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia), using conventional cytogenetic approaches, immunostaining of meiotic chromosomes, fluorescence in situ hybridization as well as comparative genomic hybridization (CGH). We found that the two nightingale species show conserved karyotypes with the same diploid chromosome number of 2n = 84. In addition to standard chromosomes, both species possessed a small germline restricted chromosome of similar size as a microchromosome. Just a few subtle changes in chromosome morphology were observed between the species, suggesting that only a limited number of chromosomal rearrangements occurred after the species divergence. The interspecific CGH experiment suggested that the two nightingale species might have diverged in centromeric repetitive sequences in most macro- and microchromosomes. In addition, some chromosomes showed changes in copy number of centromeric repeats between the species. The observation of very similar karyotypes in the two nightingale species is consistent with a generally slow rate of karyotype evolution in birds. The divergence of centromeric sequences between the two species could theoretically cause meiotic drive or reduced fertility in interspecific hybrids. Nevertheless, further studies are needed to evaluate the potential role of chromosomal structural variations in nightingale speciation.
Highlights
Despite an increasing number of sequenced avian genomes (Jarvis et al, 2014; Zhang et al, 2014; Feng et al, 2020), we still know relatively little about the organization of the genomes into chromosomes and to what degree the chromosomal structure varies among species
We found that the two species have the same diploid chromosome number 2n 84 and both possess a micro germ-line restricted chromosome (GRC) in the germ cells
A few subtle changes in chromosome morphology imply that some chromosomal rearrangements might have occurred between the species
Summary
Despite an increasing number of sequenced avian genomes (Jarvis et al, 2014; Zhang et al, 2014; Feng et al, 2020), we still know relatively little about the organization of the genomes into chromosomes and to what degree the chromosomal structure (i.e., number, size and collinearity of chromosomes) varies among species. Chromosomal translocations may cause problems with chromosome pairing, recombination and segregation during meiosis, which can lead to hybrid sterility (White, 1978; King, 1993; Homolka et al, 2007) Structural changes, such as inversions, may facilitate speciation by reducing the recombination rate within the structural variant, which may help to maintain the species-specific traits in the face of gene flow (Rieseberg, 2001; Ortíz-Barrientos et al, 2002; Butlin, 2005; Hoffmann and Rieseberg, 2008). Despite the assumed importance of structural variants in speciation, there are still relatively few studies comparing chromosomal structure between closely related species in the early stages of divergence (Hooper and Price, 2015, 2017; Hooper et al, 2019; Weissensteiner et al, 2020)
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